System and method for modulating the pacing rate based on patient activity and position

ABSTRACT

A system and method are provided for compensating for the drop in blood pressure upon standing. Upon transition from prolonged sitting, lying down, or standing position, the pacemaker abruptly increasing its pacing rate upon postural transition. This pacing rate is triggered when a patient stands after a prolonged reclined or supine/prone position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 09/359,025, filed Jul. 22, 1999 now U.S. Pat. No.6,351,672.

FIELD OF INVENTION

This invention relates to implantable cardiac stimulation devices whichmonitor the activity level of a patient to detect changes in activitythat indicate changes in body position and metabolic need and varies thestimulation rate as needed.

BACKGROUND OF THE INVENTION

A pacemaker is an implantable stimulation device that deliverselectrical stimulation pulses to cardiac tissue to relieve symptomsassociated with bradycardia, a condition in which a patient cannotnormally maintain a physiologically acceptable heart rate. Earlypacemakers delivered stimulation pulses at regular intervals in order tomaintain a predetermined heart rate, typically a rate deemed to beappropriate for the patient at rest.

Early advances in pacemakers included the ability to sense a patient'scardiac rhythm. This led to the development of demand pacemakers, sonamed because they deliver stimulation pulses only as needed by theheart. Demand pacemakers are able to detect spontaneous, hemodynamicallyeffective cardiac contractions that occur within an acceptable timeperiod. This extends the life of the pacemaker's battery as well asavoids competition with the heart's intrinsic rhythm.

The next major advance in pacemakers included the rate-responsivepacemaker which automatically adjusts the patient's heart rate inaccordance with metabolic demands. An implanted rate-responsivepacemaker typically operates to maintain a predetermined base rate whena patient is engaged in physical activity at or below a threshold leveland gradually increases the paced heart rate in accordance withincreases in physical activity until a maximum rate is reached. Thesepacemakers typically correlate measured physical activity to anappropriate heart rate and define a transition slope between the minimumand maximum heart rate. This transition slope can be telemetricallyadjusted to meet patient needs. A common rate-responsive sensor is anactivity sensor that transduces mechanical forces associated withphysical activity into an electrical signals. Typically, these activitysensors generally contain a piezoelectric transducing element whichgenerates a measurable electrical potential. The pacemaker then analyzesthis signal to determine the stimulation rate.

A variety of signal-processing techniques have been used to process theraw activity sensor signals. In one approach, the raw signals arerectified and filtered. Also, the frequency of the highest signal peakscan be monitored. Typically, the end result is a digital signalindicative of the level of sensed activity at a given time. The activitylevel is then applied to a transfer function which defines the pacingrate (also known as the sensor indicated rate) for each possibleactivity level. Attention is drawn to U.S. Pat. No. 5,074,302 to Pooreet al., entitled “Self-Adjusting Rate-Responsive Pacemaker and MethodThereof”, issued Dec. 24, 1991, which is hereby incorporated byreference in its entirety. This transfer function can be modifiedtelemetrically by the patient's physician. It can also be modifiedwithin the pacemaker based upon the stored history of the patient'sactivity levels to define a new transfer function.

While the rate-responsive pacemaker has very closely mimicked thefunction of a normal heart during exercise, it was discovered that somepatients could not sleep well because either the base rate was too highor they were experiencing short bursts of increased stimulation rate,possibly from sleep movement, that would waken them. This base rate didnot accommodate the patient's need for a lower stimulation rate duringsleep or sustained rest. A 10–20 beats per minute (bpm) difference canresult in difficulty sleeping as well as unnecessarily depleting thepacemaker battery. An example of a rate-responsive pacemaker, whichdetermines when a patient is sleeping and adjusts its base rateaccordingly, is set forth in U.S. Pat. No. 5,476,483 to Bornzin et al.,entitled “System and Method for Modulating the Base rate During Sleepfor a Rate-Responsive Cardiac Pacemaker”, issued Dec. 19, 1995, which ishereby incorporated by reference in its entirety. This reference setsforth methods of modulating the base rate based upon the monitoring ofactivity variance. By monitoring the variance of an activity signal, ithas been shown that one can distinguish between sleep (low variance inthe activity signal) and exercises (high variance in the activitysignal). This modulated base rate is also known as the circadian baserate. Otherwise, the processor uses the activity transfer function asdefined above to determine the stimulation rate.

Unfortunately, there is another group of patients who are not fullyassisted with the above stimulation methods. These patients aretypically long-term sufferers of diabetes. These individuals have atendency to gradually lose the function of their autonomic nerves. Thisis caused by a widespread degeneration of the neurons in the brain andspinal cord due to long term exposure to excess levels of blood sugar.This condition is characterized by a marked decrease in blood pressureupon standing caused by an inability to increase the heart rate andconstrict the systemic resistance and capacitance vessels. Healthyindividuals, in contrast, can increase their heart rate immediately whenthey are standing from a prolonged reclined or sitting position. Thisnormal response is called orthostatic compensation. As a result, thispatient group has a need for a pacemaker which detects their change inbody position from lying or sitting to standing and compensates with anabrupt increase in the pacing rate.

Many different methods have been attempted to determine the physicalposition of the patient. An example is U.S. Pat. No. 5,354,317 to Alt,entitled “Apparatus and Method for Cardiac Pacing Responsive to PatientPosition”, issued Oct. 11, 1994, in which the controller monitors amotion sensor to produce a static output which represents the staticposition of the patient, i.e. lying down or upright. This static outputis used to determine which of the predetermined base rates should beused, i.e. the sleep base rate or the awake base rate. This reference,however, depends upon the detection of a static and stable position. TheDC accelerometer, for example, cannot differentiate between the patientlying on the left or right side and standing. It also cannot detect thedifference between standing and sitting.

What is needed for this patient population, suffering from a lack oforthostatic compensation, is an abrupt increase in the pacing rate. The'317 reference only teaches to increase the pacing rate upon standingfrom lying down supine or prone, but does not include standing fromsitting or standing from lying on one's side. In addition, the '317reference depends upon the detection of a static position from aposition sensor to determine when to implement the stimulation therapy.The '317 reference also fails to teach the adjustment of the pacing rateto accommodate the patient's sleep cycle.

Accordingly, it is desirable to develop an implantable cardiacstimulation device which maintains the patient's heart rate in relationto the activity level or other metabolic indicator and detects the needfor an abrupt increase in the pacing rate upon standing from sitting orlying, thereby mimicking the normal heart's response to orthostaticcompensation.

SUMMARY OF THE INVENTION

The present invention is directed towards an implantable stimulationdevice that monitors a patient's activity level and in particular tochanges in that activity level to determine when a patient has a suddenincrease in activity after an extended period of inactivity. Meetingthese conditions indicates that the patient must be standing, and thestimulation device compensates for the sudden drop in blood pressureupon standing (also known as the orthostatic compensation pacing).

To this end, the present invention is directed toward any stimulationdevice that uses a sensor to detect an indicator of patient activityover time. While the preferred embodiment is directed toward a single ACaccelerometer, other types of sensors may be used, such as oxygensaturation sensors; impedance sensors that measures the change in bloodvolume; and sensors that detect the change in IEGM or evoked response,etc. The signals from the AC accelerometer or other sensor are then usedto derive the activity level signal and the long term variance inactivity. These two indicators are used to determine when a patient mustbe standing after a prolonged period of time in the sitting or lyingposition.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 shows a simplified functional block diagram of an implantablestimulation device, according to the present invention;

FIG. 2 shows a manner in which the pacing rate can be modulated in lightof activity levels;

FIG. 3 shows the response of a normal heart upon standing,

FIG. 4 shows a pacing rate for the orthostatic compensation pacingmethod, according to the present invention; and

FIG. 5 shows a flow chart of determining whether the pacing rate followsthe activity indicated rate, circadian base rate, or the orthostaticcompensation pacing rate method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the implantable cardiac stimulation device inaccordance with this invention is shown as a dual sensor rate-responsivepacemaker 10. It is well within the scope of this invention to operatethis stimulation device in a demand mode as is well known within theart. While the preferred embodiment is directed towards a stimulationdevice which uses an activity sensor for determining the pacing rate, itis well within the scope of this invention to apply the principles ofthis invention for use with other physiologic sensors that measuremetabolic demand.

FIG. 1 sets forth a simplified block diagram of the stimulation device10. The stimulation device 10 is coupled to a heart 5 by way of twoleads 12, 14. The first lead 12 has at least one electrode 18 in contactwith the atrium of the heart 5, and the second lead 14 has at least oneelectrode 20 in contact with the ventricles of the heart 5. The leads12, 14 are electrically and physically connected to the stimulationdevice 10 through a connector 16 which forms an integral part of thehousing (not shown) in which the circuits of the stimulation device 10are housed. The connector 16 electrically protects circuits within thestimulation device 10 via a protection network 17 from excessive shocksor voltages that could appear on electrodes 18, 20 in the event withcontact with a high voltage signal, e.g., from a defibrillator shock.

The leads 12, 14 carry the stimulating pulses to the electrodes 18, 20from an atrial pulse generator 22 and a ventricular pulse generator 24,respectively. Further, the electrical signals from the atrium arecarried from electrode 18 through the lead 12 to the input terminal ofan atrial channel sense amplifier 26. The electrical signals from theventricle are carried from the electrode 20 through the lead 14 to theinput terminal of the ventricular channel sense amplifier 28. Similarly,electrical signals from both the atrium and ventricle are applied to theinputs of the IEGM (intracardiac electrogram) amplifier 30. Thestimulation device 10 detects an evoked response from the heart inresponse to an applied stimulus, allowing the detection of capture witha suitable broad bandpass filter. The IEGM amplifier 30 is also usedduring transmission to the external programmer's 60 state machine orother control logic.

The stimulation device 10 is controlled by a controller 32 thattypically includes a microprocessor to carrying out the control andtiming functions. The controller 32 receives output signals from theatrial amplifier 26, the ventricular amplifier 28, and the IEGMamplifier 30 over the signal lines 34, 36, 38, respectively.

The controller 32 also generates trigger signals that are sent to theatrial pulse generator 22 and the ventricular pulse generator 24 overthe signal lines 40, 42, respectively.

The stimulation device 10 also includes a memory circuit 48 that iscoupled to a control system 32 over a suitable data/address bus 50. Thismemory circuit 48 allows certain control parameters, used by thecontroller 32 in controlling the operation of the stimulation device 10,to be stored and modified, as required, in order to customize thestimulation device's operation to suit the needs of a particularpatient. Further, the data sensed by the IEGM amplifier 30 during theoperation of the stimulation device 10 may be stored in the memorycircuit 48 for later retrieval and analysis.

A clock circuit 52 directs appropriate clock signal(s) to the controller32 as well as any other circuits throughout the stimulation device 10,e.g., to the memory circuit 48, by the clock bus 54.

The stimulation device 10 also includes a telemetry communicationscircuit 56 which is connected to the controller 32 by way of a suitablecommand/data bus 58. In turn, the telemetry circuit 56 is selectivelycoupled to an external programming device 60 by an appropriatecommunication link 62 such as any suitable electromagnetic link.Advantageously, through the external programmer 60 and the communicationlink 62, desired commands may be sent to the control system 32. Otherdata measured within or by the stimulation device 10 such as IEGM data,etc. may be stored and uploaded to the programmer 60.

The stimulation device 10 additionally includes a battery 64 whichprovides operating power to all the circuits of the stimulation device10 via a POWER signal line 66.

The stimulation device 10 also includes a sensor 68 that is connected tothe controller 32 over a suitable connection line 72. In the preferredembodiment, this sensor detects patient activity via an ACaccelerometer, but could be any appropriate sensor which can indicatepatient activity. Attention is directed to '483 for further examples ofother suitable activity sensors. In addition, any sensor which indicatesmetabolic need over time could be used in place of the activity sensor.Such sensors could be an oxygen saturation sensor, temperature sensor,etc. In the case of these alternative sensors, the sensor would beplaced on the lead 14 as shown by alternative sensor 69.

The operation of the above described stimulation device 10 is similar tothe conventional manner to provide pacing pulses at a rate thatcomfortably meets the patient's metabolic demands. In this case, thecontroller 32 uses signals generated by the sensor 68 to determine theactivity level of the patient. The measured level of activity isindicative of metabolic need. Many methods of determining the activitylevel are well known in the art. Attention is drawn to '483, which ishereby incorporated by reference.

To regulate the pacing rate, the controller 32 provides a trigger signalto the A-pulse generator 22 and/or the V-pulse generator 24. The timingof this signal (to a large extent) is determined by the activity level.

In FIG. 2, the activity indicated rate and the circadian base ratemethods are illustrated together via a single transfer function. Thetransfer function is used by the control system to correlate theactivity level measurements shown along the horizontal axis to theactivity indicated pacing rates shown along the vertical axis. Thecontroller 32 then triggers the appropriate pulse generator 22, 24 atthe activity indicated rate. It should be noted that an appropriatetransfer function can be used based upon individual patient need. Inaddition, different modes of pacing (i.e. DDD, VVI, etc.) can beaccommodated by this method.

Two activity levels are noted on the horizontal axis of the transferfunction: a low activity threshold 202 and a high activity threshold204. For activity level measurements above the high activity threshold204, the pacing rate is maintained at a maximum pacing rate 200. Foractivity level measurements between the low activity threshold 202 andthe high activity threshold 204, the activity indicated pacing ratevaries according to the programmed transfer function 208. In this case,the activity indicated pacing rate varies linearly between a circadianbase rate 210 and a maximum pacing rate 200. However, this transitioncan be programmed to meet patient's needs by the physician or adjustedby the processor periodically as set forth in U.S. Pat. No. 5,514,162 toBornzin et al, entitled “System and Method for Automatically Determiningthe Slope of a Transfer Function for a Rate-Responsive CardiacPacemaker”, issued May 7, 1996, hereby incorporated by reference.

For activity level measurements below the low activity threshold 202,the processor sets the pacing rate to a rate defined by the circadianbase rate 210. The circadian base rate 210 is defined through monitoringthe activity variance measurements as described more fully in '483. Apatient's activity levels are monitored and activity variancemeasurements are calculated and monitored to determine when and how longa patient typically sleeps. These two terms are used to define astimulation rate which is below the programmed base rate of thestimulation device 10 such that the patient receives a lower pacing rateduring sleep. This lower pacing rate more closely mimics the naturalcardiac rhythm.

As discussed above, the patient group which suffers from long termdiabetes tends to develop neuropathy from the long term exposure oftheir nerves to excessive blood sugar levels. This condition erodes theability to adequately control the heart rate. In particular, thiscondition render the patient unable to compensate for the dramatic dropin blood pressure upon standing after sitting or lying down due to aninability to increase the heart rate and constrict the system resistanceand capacitance vessels.

To overcome this condition, the controller 32 compensates for the changein patient position by triggering at an orthostatic compensation pacingrate.

FIG. 3 sets forth the normal heart response to provide the orthostaticcompensation. Upon standing after a prolonged period of sitting or lyingdown, the normal autonomic nervous system abruptly increases the naturalheart rate to approximately 80–100 bpm in 3–8 seconds at 302. Then, thenatural heart rate then slowly decreases to a rest rate in 2 seconds toone minute, typically approximately 60–70 bpm at 304.

FIG. 4 sets forth the orthostatic compensation pacing method thatprovides the orthostatic compensation to a patient that does notnaturally have it. The controller 32 abruptly increases the stimulationrate to approximately 85–100 bpm in 8 seconds at 402. The upperorthostatic compensation pacing rate is determined by the clinician foreach individual patient. Then, the stimulation rate is slowly decreasedto a high base rate in 20–60 seconds, typically approximately 70 bpm at404. The controller can override the orthostatic compensation pacingmethod if the patient activity level indicates an immediate stimulationrate higher than that of the immediate orthostatic compensation pacingrate. In other words, if the activity indicated pacing rate or thecircadian base pacing rate should exceed the orthostatic compensationpacing rate at a particular time, then the controller 32 would followthe maximum of the three indicated stimulation rates. Once theorthostatic compensation pacing rate has finished, the controller 32uses the transfer function 208 as set forth in FIG. 2 to determine theimmediate stimulation rate.

To determine when a patient is in need of orthostatic compensation, thecontroller 32 uses the activity level signals and the activity variancemeasurements to determine when the patient has stood after prolongedtime of sitting or lying down.

To this end, the controller 32 monitors two main variables in relationto two preset thresholds: the immediate activity level and the long termvariance in activity (also known as the activity variance measurement).The clinician programs the activity threshold for a patient appropriateactivity level. The activity variance threshold is predetermined basedupon the patient's activity. To determine the activity variancethreshold, the activity variance measurements are stored in a histogramfor a week. Further information regarding histograms can be found in'483.

To determine when a patient is standing after an extended period ofinactivity, a night time activity variance threshold and a day timeactivity variance threshold are determined. The night time activityvariance threshold is predetermined to be the bins of the histogramwhich contain counts of less than 17.5% of the total histogram counts.The day time activity variance is predetermined to be the bins in thehistogram which contains counts of less than 67.5% of the total bincount. These two activity variance thresholds are then averaged todetermine an activity variance threshold that indicates a patient isresting.

In addition to monitoring the patient activity and the activity variancemeasurements, the controller 32 monitors how long the activity variancemeasurement is below the activity variance threshold. This time duration(T_(rest(expired))) is the necessary time for a patient to be restingand then in need of orthostatic compensation once the patient stands. Ithas been found that a T_(rest(expired)) time duration of 1 minute to 3hours as programmed by the clinician is the time duration thatadequately indicates when a patient will require orthostaticcompensation pacing upon standing.

FIG. 5 sets forth the flow chart describing how the controller 32determines the immediate pacing rate and if patient has changed bodypositions and has previously been sitting or lying down for an extendedperiod of time.

Following Flow 1, the controller 32 determines if the activity levelsignal is less than an activity threshold (Block 502), and if theactivity variance measurement is greater than an activity variancethreshold (Block 504). If these conditions are met, then the patientmust be inactive, and the controller 32 adjusts the pacing rate tofollow the circadian base rate (Block 506) as set forth in FIG. 2.

Following Flow 2, the controller 32 determines if the activity levelsignal is less than the activity threshold (Block 502), and if theactivity variance is also less than the activity variance threshold(Block 504). If these conditions are met, then the patient must be in areclined or in a supine/prone position and is resting. At this point,the controller 32 starts a duration timer. This duration timer monitorsthe time the patient is at an extended period of inactivity (i.e.T_(rest)) (Block 508). The controller 32 adjusts the stimulation rate tofollow the circadian base pacing rate (Block 506) as set forth in FIG.2.

Following Flow 3, the controller 32 determines if the activity levelsignal is greater than an activity threshold (Block 502) and if theT_(rest) duration timer has not been started (i.e. T_(rest)=0)(Block520). If these conditions are met, then the patient must be active andmust have been active. In this case, the controller 32 adjusts thestimulation rate to be the maximum of the activity indicated pacing rateand the circadian base pacing rate (Block 522) as set forth by thetransfer function 208 in FIG. 2.

Following Flow 4, the controller 32 determines if the activity levelsignal is greater than an activity threshold (Block 502) and if theT_(rest) duration timer has been started (Block 520) but has notexceeded T_(rest(expired)) (Block 524). If these conditions are meet,then the patient has been resting a short period of time and has stood.In this case, the controller resets the T_(rest) duration timer (Block526) and adjusts the pacing rate to the maximum of the activityindicated pacing and the circadian base pacing rate as set forth by thetransfer function 208 (Block 522).

Following Flow 5, the controller 32 determines if the activity levelsignal is greater than an activity threshold (Block 502), and theT_(rest) duration timer is greater than the T_(rest(expired)) duration(Blocks 520, 524). If these conditions are met, then the patient musthave just stood and is in need of orthostatic compensation pacing. Theorthostatic compensation pacing method has a total regimen time (N) 406,and the controller 32 starts a regimen duration timer to monitor how farinto the orthostatic compensation pacing method the patient is (K) 408.If the regimen duration timer indicates that the pacing method is notcomplete (K<N) (Block 528), then the controller 32 adjusts thestimulation rate to be the maximum of the activity indicated pacingrate, circadian base pacing rate, and the orthostatic compensationpacing rate (Block 530), and increments the regimen duration timer(Block 532) (i.e. K=K+1).

Following Flow 6, the controller determines if the activity level signalis greater than an activity threshold (Block 502), the T_(rest) durationtimer exceeds T_(rest(expired)) time period (Blocks 520, 524), and theregimen duration timer exceeds the regimen time (K>N) (Block 528). Ifthese conditions are met, then the patient must have just stood and hasfinished orthostatic compensation pacing. At this point, the controller32 resets the T_(rest) duration timer and the regimen duration timer(Block 540) and adjusts the stimulation rate to be the maximum of theactivity indicated pacing rate and the circadian base rate (Block 522)as set forth in FIG. 2.

Although the invention has been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of the application of the principles of the invention. Forinstance, this method can be used to detect instantaneous changes andlong term variations in any signal indicative of metabolic need to varythe pacing rate and pacing method as described. Such sensors could be anoxygen saturation sensor, impedance sensors that measures the change inblood volume; and sensors that detect the change in IEGM or evokedresponse, etc. Accordingly, the embodiments described in particularshould be considered exemplary, not limiting, with respect to thefollowing claims.

1. In an implantable cardiac stimulation device, a method of determininga pacing rate, the method comprising: monitoring an indicator of patientactivity and generating corresponding signals; processing the signals todetermine a patient's activity level; monitoring the patient's activitylevel for a predetermined change in the activity level; and pacing at anorthostatic compensation pacing rate if the predetermined change in theactivity level is sensed.
 2. The method of claim 1, wherein: monitoringthe patient's activity level comprises determining a patient activitylevel and an activity variance measurement from the activity signal. 3.The method of claim 2, wherein pacing at the orthostatic compensationpacing rate is performed if the activity variance measurement is below afirst predetermined threshold for a predetermined time period, followedby the activity level signal exceeding a second predetermined threshold.4. The method of claim 1, wherein the orthostatic pacing rate abruptlyincreases the pacing rate to between about 80 and about 100 beats perminute and then slowly decreases the pacing rate over a period of about20 seconds to one minute.
 5. The method of claim 1, wherein monitoringthe patient's activity level comprises monitoring for a period ofinactivity followed by an activity level that exceeds a predeterminedthreshold.
 6. The method of claim 1, wherein monitoring the indicator ofpatient activity comprises using at least one of an AC accelerometer, anoxygen saturation sensor, an impedance sensor, and a sensor that detectsa change in at least one of an intracardiac electrogram and an evokedresponse signal.
 7. An implantable cardiac stimulation devicecomprising: means for monitoring an indicator of patient activity andfor generating corresponding signals; means for generating stimulationpulses; and means for processing the signals to determine apredetermined change in patient activity level, and for implementing anorthostatic compensation therapy based on detecting the predeterminedchange, the means for processing comprising means for controlling themeans for generating according to the orthostatic compensation therapy.8. The implantable stimulation device of claim 7, wherein: means forprocessing further comprises means for implementing an orthostaticcompensation therapy that abruptly increasing the pacing rate, followedby slowly decreasing the pacing rate.
 9. The implantable stimulationdevice of claim 7, wherein the means for monitoring comprises at leastone of an AC accelerometer, an oxygen saturation sensor, an impedancesensor, and a sensor that detects a change in at least one of anintracardiac electrogram and an evoked response signal.
 10. Theimplantable stimulation device of claim 7, wherein: the means forprocessing is operative to determine the need for orthostaticcompensation therapy when the patient activity is below a firstthreshold for a predetermined time period, followed by the patientactivity level exceeding a second threshold.
 11. The Implantablestimulation device of claim 7, further comprising means for triggeringpacing pulses, when the patient is not in need of orthostaticcompensation therapy, at a pacing rate as determined from the sensorsignals.